1. Field of the Invention
This invention relates generally to optical switching systems, and more particularly, to a method and apparatus for routing input signals into all possible output combinations by using waveguide grating-based wavelength selective switch modules.
2. Description of the Related Art
Current optical switching and signal transmission systems are limited to optical switching of an entire spectrum without wavelength differentiation or selection. Due to the lack of wavelength selection, an optical switch must frequently operate in conjunction with a demultiplexer and multiplexer to achieve routing of optical signals having different wavelengths to different ports. This requirement leads to more complicated system configurations, higher manufacture and maintenance costs, and lower system reliability. For this reason, even though optical switches are advantageous because the optical signals are switched entirely in the optical domain without converting them into the electrical domain, the cost and size of such optical switches can be prohibitive for many applications.
Thus, there is a need to further improve optical switches, since they are considered critical enabling technology for optical-fiber networks. In the wavelength division multiplex (WDM) networks of the past, the adding, dropping or cross-connecting of individual wavelengths has involved conversion of the signal into the electrical domain. Development of all-optical switches for applications ranging from add-drop functionality to large-scale cross-connects is key to efficient optical networking systems. However, with current technical limitations, an all fiber network implemented with optical switches are still quite expensive.
The primary optical switching technologies being developed today are: micro electromechanical systems (MEMS), liquid crystals, thermal-optics, holograms, acousto-optic, etc. Among all these optical switching technologies, MEMS is emerging to be the most promising technology, thanks to its potential for economical mass production, as well as its reliability in a wide range of applications. The other technologies are still in the experimental stage and will require years of development to become reliable enough for commercial applications.
There are two types of optical MEMS switch architectures under development, or commercially available: mechanical and micro-fluidic. Mechanical-type MEMS-based switches use arrays of miniaturized mirrors fabricated on a single chip. The optical signal is reflected off this tiny mirror in order to change the transmission channel. Micro-fluidic-type MEMS-based switches, on the other hand, have no moving mirrors. Rather, they rely on the movement of bubbles in micro-machined channels.
Mechanical-type MEMS-based switches can be further classified into two categories according to mirror movement: two-dimensional (2-D) switches and three-dimensional (3-D) switches. In 2-D switches, the mirrors are only able to execute a two-position operation—that is, the mirrors can move either up and down or side by side. In 3-D switches, the mirrors can assume a variety of positions by swiveling along multiple axes. These products (2-D switches or 3-D switches) are able to offer such benefits as excellent optical performance, minimal cross-talk, and the promise of improved integration, scalability, and reliability. However, in these switches, light travels through free space, which causes unbalanced power loss. Further, in order to steer each mirror, multiple electrodes need to be connected to each mirror, which increases manufacturing complexity, particularly for large-scale mechanical-type MEMS-based switches. Finally, alignment and packaging are problematical for large-scale switches.
While the above-mentioned micro-mirror-based approach is widely pursued by many manufacturers to build their MEMS-based optical switches, Agilent Technology, Inc. has developed micro-fluidic-type, MEMS-based switches by combining its micro-fluidics and ink-jet printing technology. In these switches, an index-matching fluid is used to select and switch wavelengths. This fluid enables transmission in a first, normal condition. To redirect light from an input to another output, a thermal ink-jet element creates a bubble in the fluid in a trench located at the intersection between the input wave-guide and the desired output wave-guide, reflecting the light by total internal reflection. The advantages of these switches are that they have no moving mechanical parts and are polarization independent. However, these types of switches have not been proven to be completely reliable. Further, these switches often result in insertion loss for large-scale switches.
A common drawback of both of these two types of MEMS-based switches is the requirement to work with external de-multiplexing and re-multiplexing systems in order to function properly in an optical networking system. The requirements of implementing de-multiplexing and re-multiplexing functions add tremendous complexities to the system configuration and significantly increase the cost of manufacture, system installation, and maintenance of the optical network systems. Another drawback of both of these two types of MEMS-based switches is that these prior art switching systems are not wavelength selective switches. In other words, the switching systems cannot selectively switch a particular wavelength from an input waveguide to a desired output waveguide. In short, they are not wavelength discriminating devices.
In order to have wavelength discrimination, a Bragg grating has been shown to have excellent wavelength selection characteristics. A Bragg grating behaves as a wavelength-selective filter, reflecting a narrow band of wavelengths, while transmitting all other wavelengths. The Massachusetts Institute of Technology (MIT) has developed a technology for building Bragg grating devices in planar optical waveguides. These so-called integrated Bragg gratings offer many advantages over the fiber Bragg grating, according to MIT.
Therefore, a need exists to provide an innovative method for constructing MEMS-actuated highly integrated wavelength selective switches. It is desirable that the improved optical switch be able to eliminate unbalanced power loss, be simple to manufacture, have low insertion loss and power consumption, and be reliable.
Current optical switch systems have serious drawbacks and practical limitations. An example is the optical switch system disclosed in U.S. Pat. No. 6,253,000, in which the building block is a traditional multiport coupler. Drawbacks of this type of switch system include: (1) a large number of couplers are required to scale up the matrix and (2) the insertion loss of signals at various input ports varies greatly. A similar example is described in U.S. Pat. No. 6,208,778. Therefore, a need exists to provide a wavelength intelligent optical switch capable of routing various incoming wavelengths and also capable of scaling up with a relatively simple yet flexible structure. Once fully developed, they will be the building block of various modules used in the optical communication network.